1. Field of the Invention
This invention relates to an exhaust gas-purifying system for internal combustion engines.
2. Prior Art
In the technical field of internal combustion engines, it is well known that a catalyzer (three-way catalyst) is arranged in the exhaust system of the engine, for purifying noxious components, such as HC, CO, and NOx present in exhaust gases emitted from the engine. However, the above catalyzer is not activated until it is warmed up to a temperature exceeding a predetermined value, which means that the catalyzer does not perform a desired catalytic action before its temperature exceeds the predetermined value. It is therefore necessary to promptly activate the catalyzer by positively heating same when it is not warmed up, on such an occasion as the start of the engine.
To achieve early activation of a catalyzer by promptly elevating the temperature thereof, a catalyzer provided with an electric heater has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 48-54312.
in the above proposed conventional catalyzer with an electric heater, however, if the electric heater is operated when the engine is in a high load operating condition, such as a condition under which a throttle valve arranged in the intake pipe of the engine is fully open, the catalyzer is exposed to a high temperature atmosphere, resulting in deterioration of the converter and the electric heater due to heating, to thereby shorten the durability or effective lives thereof.
Besides, the electric heater provided for the above conventional catalyzer is required to have relatively large electric capacity. However, the technique of controlling an electric heater of this kind has not been established yet, and therefore, if an electric heater of this kind is mounted in automotive vehicles, it cannot exhibit satisfactory practical performance, in respect of heating efficiency, power consumption, etc.